1. Field of the Invention
The present invention relates to a display control system and a method thereof, suitable for a computer system including an LCD (liquid crystal display) or a plasma display as a standard display unit and allowing connection of a CRT display as an optional display unit.
2. Description of Related Art
Conventional portable personal computers such as laptop computers have flat panel displays such as LCD displays and plasma displays as standard display units. These flat panel displays are generally designed to display data by using gray scale levels. Although color LCDs have been developed, they can display only a small number of colors. For this reason, these computers are designed to allow connection of color CRTs (also monochrome CRTs).
Among these computers, there are computers which cannot simultaneously operate a display unit as standard display unit and a display unit connected as an optional display unit. In this case, when the power source is turned on, it is automatically checked whether an optional display unit is connected to the computer. More specifically, when the power source is turned on, an initialization and reliability routine of a basic input/output system (BIOS) is executed to check whether an optional display unit is connected. If the optional display unit is connected, data is displayed on the optional display unit. If the optional display unit is not connected, data is display on the standard display unit.
In a display control system such as a VGA (video Graphic Array) adapter, when it is to be checked whether an optional display unit is connected, specific data for determining the connection of a CRT display is written in a video RAM, the specific data read out from the VRAM is converted into an analog value by a D/A converter, and it is determined, on the basis of loads connected to the R, G and B terminals of the DAC, whether the CRT is connected.
If, however, a resume mode (in which system data indicating the operation conditions of a CPU is saved in response to a power-off signal, and the saved data is restored in response to a power-on signal) is set in a setup process, the operation conditions of the system (the register data, processed data, status data, and the like of the CPU) set immediately before the power-off are saved in a backup memory, and the data saved in the backup memory is restored when the power is supplied again, so that the process interrupted upon power-off is resumed. For this reason, if the power source is turned off and on in the resume mode, the above-mentioned initialization and reliability routine is not executed, and hence it is not determined whether the optional display unit is connected. Therefore, in a case wherein the optional display unit is selected by a setup process while the resume mode is set, if the power source is turned off, and is turned on again after the optional display unit is detached from the computer main body, the resume function works, and an attempt is made to display data on the optional display unit. Consequently, the data is not displayed on either the optional display unit or the standard display unit.
In addition, original data in the VRAM is destroyed by writing the specific data in the VRAM, the original displayed data cannot be reproduced. That is, the resume mode is not achieved.